In data readout systems, such as utilized with disk drives for example, data bits are preferably closely packed on the storage medium during recording in order to achieve high density storage. A conventional approach for detecting the recorded data signal is by sensing the peaks of the data bits which appear within allotted time cells or positions. However, during readout detection of the recorded signal, errors arise due to noise, interference and dropouts, among other things, particularly more so when recorded bits are closely packed and the time cells for recognizing valid signals are relatively narrow.
In virtually all peak-position detection systems of this type, the readout signal is differentiated to change a peak into a zero crossing. The differentiated signal is applied to a voltage comparator that acts as a zero crossing detector, thereby producing a sharp transition at its output whenever the input signal passes through zero.
One problem that arises with a zero crossing detector of this type is that if the input signal returns to the baseline between peaks, the differentiated signal will return to zero. However, since the input signal has been differentiated, there will be considerable noise on its baseline. This noise results in many extraneous zero crossings which are not derived from a valid peak of the input signal.
In readout systems for sensing data bits which have been stored on a magnetic medium for example, amplitude threshold detection of the input signal is often employed for validating zero crossing data signals. Amplitude threshold detection may be accomplished by processing the differentiated input signal or by using the undifferentiated signal at the input to a threshold detector. The application of a threshold to the undifferentiated signal works well for signals having wide spacing between peaks, and thus experiences very little inter-symbol-interference. However, where the data bits are closely spaced and there is substantial inter-symbol-interfernce, the amplitude of the closely spaced peaks of the undifferentiated signal will result in the threshold detector becoming sensitive to small variations in the signal amplitude.
In contrast when inter-symbol-interference occurs in the differentiated signal, the amplitude at the point of interference tends to increase, so that applying the differentiated signal to the threshold detector results in improved performance for closely spaced data bits. However, the system experiences problems qualifying isolated data bits, particularly if the pulses are poorly shaped, such as unwanted shoulders found with some pulses, due to a defect on the magnetic recording medium.